Sensei

Sensei (先生) is the human who created the Servamps. He was killed by Sleepy Ash hundreds of years ago before the series.

Appearance
While his appearance in the manga hasn't been shown, in the anime, he closely resembles Kuro, having short bluish white hair, wearing white shirt, grey pants, a pair of black boots, and white haori over his body.

Background
Sensei was a researcher born hundreds of years ago who was referred as "The Count". He studied magicians and various supernatural creatures, mainly vampires. His research attracted many magicians, gathering them together. This led to the creation of a group of magicians called Conjurer Control Convention, otherwise presently known as C3.

His research eventually resulted him capable of changing seven humans into a special vampires known as Servamps that each represents the seven deadly sins. However, not all Servamps were happy with their change, particularly Sleepy Ash of Sloth who despite having a close relationship with Sensei, views the existence of monsters such as vampires are unnecessary, and All of Love of Lust who view his change as a punishment since he doesn't wish to live anymore yet was turned into an immortal being.

Despite being a human, he was able to live for a hundred years without aging. During this time, he continued his research in Japan and then changed another human into a Servamp named Tsubaki who represents Melancholy. Since the other Servamps doesn't know of their existence prior their meeting with Tsubaki, it's presumed that Sensei never told any of them about their youngest sibling and kept his existence hidden from them. Furthermore, unlike the other Servamps, Tsubaki was created with the ability to break the contract item that constitutes the link between Servamps and Eves. One day, a neutral organization, C3, sent a request to the seven Servamps, which was to kill their creator. The opinions between the Servamps were divided, so they decided to do a majority vote. The Mother of Wrath, Lawless of Greed, and World End of Gluttony objected due to their attachment to their creator, while Old Child of Pride, Doubt Doubt of Envy, and All of Love agreed out of fear that their creator may create more monsters like them in the future. Sleepy Ash, who held the final vote, also thinks that their creator's research is too dangerous and has to be stopped, so he decided to personally kill their creator. Arriving in Japan, Sleepy Ash confronted Sensei in his house, and Sensei greeted him warmly. Despite his uncertainty, Sleepy Ash still sticked with his decision and killed his creator immediately after he arrived.

Sensei's death left a great impact on the Servamps, namely Sleepy Ash, Lawless, and Tsubaki. Sleepy Ash soon regretted his action afterwards, Lawless came to resent Sleepy Ash for his decision, and Tsubaki became obsessed in finding his killer and fulfil what he believed to be Sensei's expectation to break the other Servamps and fix their messed-up world.

Sleepy Ash
The Creator of the Servamps is the one who made Sleepy Ash a Servamp. Judging from what the other Servamps said, the creator was supposed to be a special person for Sleepy Ash, implying that Sleepy Ash and the creator used to be close. However, their relationship seems to have strained after he turned Sleepy Ash into a vampire, something that Sleepy Ash never wanted. Despite deciding to kill his creator, Sleepy Ash made it clear that he doesn't hate his creator, only making the decision because he thought it's necessary. This is further evident when Sleepy Ash was hesitant and even crying as he killed him. The act left a feeling of uncertainty within Sleepy Ash, and soon became regret that continued to haunt him for hundreds of years afterwards, making him questioning his own action. This shows that despite thinking that he is dangerous, Sleepy Ash actually still cares for his creator and regretted for immediately choosing to kill him rather than speaking to him first.

Tsubaki
Tsubaki seems to have a close relationship with the creator of the Servamps that he refers to as "Sensei".

One night, Tsubaki witnessed Sleepy Ash in his black lion form after he killed their creator, and Tsubaki seemed devastated at his creator's death. Since then, he has been looking for the real identity of the black lion to question why he killed their creator. He is also determined to fulfil his teacher's expectation to break the other Servamps and fix their messed up world, though it's not yet confirmed if Sensei really requested Tsubaki to do so or not.

Sensei's death resulted in Tsubaki's one-sided hatred against Sleepy Ash for killing the creator of the Servamps and not knowing him being his youngest brother like his other siblings.

The Other Servamps
Being the one who created the Servamps, the Creator was an important person for each of the Servamps, but their opinions regarding him are divided. Old Child, Doubt Doubt, and All of Love deemed him too dangerous to be left alive, fearing that he may create hundreds or even thousands of monsters like them. Lily, in particular, resented the fact that he was turned into a vampire, considering it as a punishment rather than being grateful of the creator. For this reason, they opted for him to be killed and didn't have any regret from their decision as they think it's necessary.

Lawless, World End, and The Mother, on the other hand, respect him enough that they strongly objected when received a request to kill him. Even when faced by the fact that their creator was able to live for hundreds of years, they still considered him as a human being. Lawless, in particular, seems to be in a good relationship with him as he considers their Creator as a parental figure. After their Creator was killed by Sleepy Ash, World End and The Mother reluctantly accepted the outcome but didn't blame Sleepy Ash for it. Lawless, however, hated his oldest brother for years to come. Only until Licht appears and help him to let go of chasing after his creator's shadows, finally coming to terms with his death while still remembering the man who created the Servamps.

Trivia

 * In Japanese, Sensei (先生) means master or teacher
 * It is unknown if Sensei and Sleepy Ash/Kuro are relative. Since Sensei had short bluish white hair and closely resembles Kuro, it is possible that Sensei and Sleepy Ash/Kuro are relative.
 * Sensei created C3 in England.